Question

Is it necessary to have selector’s name in same case in css and html?

is it ok ?

<div id="HEADER"> </div>

#header{...}

or this is necessary?

<div id="HEADER"> </div>

#HEADER{...}

Answer 1

Class names and IDs are case sensitive, so they must have the same casing.

Therefore, div.MyClass is not the same as div.myClass.

Element (in HTML) and pseudo-class (everywhere) names are not case sensitive, so DIV:hover is the same as div:HOVER.

For more information, read the spec.